To address the world's increasing energy requirements, efficient and environmentally sound alternatives to the use of fossil fuels are sought after. Alternative fuels, such as ethanol or biodiesel, can be produced from plant biomass. For example, the key ingredient used to produce ethanol from current processes is termed fermentable sugar. Most often, fermentable sugar is in the form of sucrose, glucose, or high-fructose corn syrup. Plants currently grown to produce such biomass include corn, sugarcane, soybeans, canola, jatropha, and so forth. But much of the plant biomass used to produce fermentable sugar requires extensive energy-intensive pre-processing. Further, use of such plant biomass can lead to soil depletion, erosion, and diversion of the food supply.
It is known that some cyanobacteria produce sucrose through the action of sucrose phosphate synthase and sucrose phosphate phosphatase, where it has been studied exclusively as an osmoprotectant. With respect to salt tolerance, cyanobacteria can be divided into three groups. Strains having low tolerance (less than 700 mM) synthesize either sucrose, as is the case with Synechococcus elongatus PCC 7942, or another dissaccharide known as trehalose [Blumwald et al., Proc Natl Acd Sci USA (1983) 80:2599-2602 and Reed et al., FEMS Microbiol Rev (1986) 39:51-56]. Glucosylglycerol is produced by strains having moderate halotolerance (0.7-1.8 mM), such as Synechocystis sp. PCC 6803. High salt tolerance (up to 2.5 M) results from the accumulation of either glycine betaine or glutamate betaine. Miao et al. [FEMS Microbiol Lett (2003) 218:71-77] determined that when glucosylglycerol biosynthesis is blocked by deletion of the agp gene, however, Synechocystis sp. PCC 6803 produces sucrose as its osmoprotectant. Desiccation tolerant cyanobacteria also produce sucrose and trehalose in response to matric water stress [Hershkovitz et al., Appl Environ Microbiol (1991) 57:645-648].
Synechocystis spp. PCC 6803 (ATCC 27184) and Synechococcus elongatus PCC 7942 (ATCC 33912) are relatively well-studied, have genetic tools available and the sequences of their genomes are known (see e.g., Koksharova, O. A. and Wolk, C. P. 2002. Appl Microbiol Biotechnol 58, 123-137; Ikeuchil, M. and Satoshi Tabata, S. 2001. Photosynthesis Research 70, 73-83; Golden, S. S., Brusslan, J. and Haselkom, R. 1987. Methods in Enzymology 153, 215-231; Friedberg, D. 1988. Methods in Enzymology 167, 736-747; Kaneko, T. et al. 1996. DNA Research 3, 109-136).
The commercial cultivation of photosynthetic microorganisms such as Spirulina maximum, Spirulina platensis, Dunaliella salina, Botrycoccus braunii, Chlorella vulgaris, Chlorella pyrenoidosa, Serenastrum capricomutum, Scenedesmus auadricauda, Porphyridium cruentum, Scenedesmus acutus, Dunaliella sp., Scenedesmus obliquus, Anabaenopsis, Aulosira, Cylindrospermum, Scenecoccus sp., Scenecosystis sp., and Tolypothrix is desirable for numerous applications including the production of fine chemicals, pharmaceuticals, cosmetic pigments, fatty acids, antioxidants, proteins with prophylactic action, growth factors, antibiotics, vitamins and polysaccharides. The algic biomass can also be useful, in a low dose, to replace or decrease the level of antibiotics in animal food or be useful as a source of proteins. Furthermore, the algic biomass provided in a wet form, as opposed to a dried form, can be fermented or liquefied by thermal processes to produce fuel. Thus, there is great interest in the ability to increase the efficiency of cultivating such organisms.
In general, current photosynthetic bioreactors rely on the cultivation of microorganisms in a liquid phase system to produce biomass. These systems are usually open-air pond-type reactors or enclosed tank-type reactors. Enclosed bioreactors, however, typically are considered to be an improvement over pond type reactors in many respects. Importantly, enclosed systems provide a barrier against environmental contamination. In addition, these systems allow for greater control of temperature and gas content of the liquid media.
Still, the uses of enclosed photobioreactors tend to be limited by photosynthetic microorganisms' requirement for light (i.e., actinic radiation provides the energy required by photosynthetic microorganisms to fix carbon dioxide into organic molecules). Thus, sufficient illumination of the photosynthetic microorganisms is an unyielding requirement. Nevertheless, as the cell density in a liquid phase photobioreactor increases, the ability of light to penetrate into the media decreases, which typically limits the cell density that may be achieved. Additionally, some type of agitation of the liquid media is generally required to prevent unwanted sedimentation of the organisms, a process that requires the input of energy.
Numerous attempts have been made to devise a method of bringing light to the organisms in liquid phase systems. For example, some systems involve circulating the liquid culture media through transparent tubes. Other attempts involve placing a light source within the media or introducing reflecting particles into the culture media to adjust the radiation absorbance of the culture. Despite these efforts, a significant increase in the ability to culture organisms in liquid phase systems at higher cell densities has not yet been achieved.
In addition to the aforementioned light requirement, the use of liquid phase photobioreactors has been burdened with providing the photosynthetic microorganisms enough carbon dioxide for photosynthesis. Typically, these systems generally incorporate some type of additional aeration system to increase the concentration of carbon dioxide dissolved in the media. Eliminating the need for aeration would greatly simplify the system thus reducing operating costs.
Liquid phase photobioreactors also tend not to be well suited for conventional methods of continuous production. In general, the transportation of large volumes of liquid is complex and burdensome. Further, because liquid phase systems usually require mechanisms for circulation, agitation, aeration, and the like, it is generally simpler and more cost effective to operate only one or a few large cultivation devices rather than numerous smaller ones. Therefore, currently practiced methods involve processing relatively large batches (i.e., a batch of photosynthetic microorganisms is cultivated and the entire resulting biomass is then harvested).
Thus, there is a great need in the art for advancement in photosynthetic bioreactor design. Providing a new type of photosynthetic bioreactor capable of efficiently cultivating and harvesting relatively high densities of photosynthetic microorganisms without large volumes of water or other liquid media, without the aforementioned extraordinary measures for supplying adequate light and carbon dioxide, and at a reasonable cost would represent a substantial advance in the art, and benefit industry and consumers alike.